newlib/.../getreent.c: Allow to be provided by host and do so for RTEMS

RTEMS provides the option to have a global or per-thread reentrancy
as part of application configuration. As part of this, RTEMS provides
the implementation of __getreent() as appropriate. Allow the target
to determine if this method is present in libc.a.

newlib/libc/reent/getreent.c

@@ -1,5 +1,11 @@
 /* default reentrant pointer when multithread enabled */
 
+#ifdef GETREENT_PROVIDED
+
+int _dummy_getreent;
+
+#else
+
 #include <_ansi.h>
 #include <reent.h>
 
@@ -12,3 +18,5 @@ _DEFUN_VOID(__getreent)
 {
   return _impure_ptr;
 }
+
+#endif